Alkalosis is a selective dilator stimulus for the pulmonary circulation. Achievement of an alkalotic pH has, for years, been a mainstay of therapy for infants with Persistent Pulmonary Hypertension of the Newborn (PPHN). At the cellular level, very little is known about the signaling mechanisms mediating alkalosis-induced pulmonary vasodilation. The long-term objective of our laboratory is the elucidation of cellular mechanisms regulating vascular tone and reactivity in the newborn pulmonary circulation. Our overall hypothesis is that there are unique endothelial-dependent signaling mechanisms at the level of the pulmonary resistance circulation that mediate the dilatory response to alkalosis. At the core of our methodology is measurement of vascular responses in pressurized and perfused pulmonary resistance vessels (PRV) isolated from newborn piglets, the first time this technique has been applied to the neonatal lung. Our preliminary data demonstrate that PRV mimic the unique responses of the intact lung to alkalosis, providing an excellent model for the study of pH-dependent vasoactive responses. This proposal will investigate the following' specific hypotheses: (a) In the pulmonary circulation, alkalosis mediates vasodilation by a mechanism which is endothelial-dependent. (b) Alkalosis activates endothelial cell K+ channels, resulting in membrane hyperpolarization and increased Ca++ influx. (c) The resulting increase in intracellular Ca++ activates the nitric oxide (NO) pathway and the prostaglandin (PG) pathway stimulating synthesis and release of NO and prostacyclin (PGI2), respectively, thus mediating vascular smooth muscle relaxation. To test these hypotheses the following specific aims will be addressed: (1) Determination of the mechanism by which the endothelium mediates the response of PRV to alkalosis. (2) Examination of the effects of alkalosis on K+ conductance, membrane potential and intracellular Ca++ concentrations ([Ca++]i) in PRV and cultured pulmonary microvascular endothelial cells. By understanding the mechanism by which alkalosis causes pulmonary vasodilation, pharmacologic strategies can be formulated to mimic, enhance or prolong this dilator effect, while minimizing the lung and brain injury associated with hyperventilation and low PCO2, respectively. It is anticipated that these studies will enhance our knowledge of the biochemical pathways regulating perinatal transitional physiology and contribute to the development of safer, more effective strategies for the treatment of infants with PPHN.